What The Science Says About Smoking Weed While Pregnant

There’s no shortage of rules when it comes to pregnancy. Don’t eat certain foods. Avoid certain medications. Don’t overdo it on the caffeine. And the most talked-about: Don’t smoke or drink.

Unfortunately for pregnant pot-lovers, marijuana and pregnancy do not mix. It might seem like an obvious guideline to avoid smoking weed while pregnant, but for a long while there wasn’t great evidence to support that advice. This is why some expecting mothers that wanted to smoke — especially if they used marijuana for a medical condition or morning sickness — often threw caution to the wind.

But now, the tides are turning. Research shows that the answer is fairly clear: Smoking weed during pregnancy is dangerous for the fetus and can lead to lower birth weight, preterm birth, and an increased risk of autism, among other conditions. And a recent study finds that using marijuana during early pregnancy is linked to an increased risk of poor pregnancy outcomes related to the placenta’s functioning, including poor fetal growth, stillbirth, and developing high blood pressure during pregnancy.

Historically, it’s been difficult for scientists to study the relationship between weed and pregnancy. Marijuana is listed as a Schedule I drug by the U.S. government, which is the same classification as heroin. Not only does that mean illegal possession can land you a felony charge, but it also means scientists have to jump through a lot of hoops to study it.

Despite these restrictions, researchers in the late 1980s figured out that the active ingredient in marijuana, THC, can travel across the bridge of the placenta and reach the fetus. And fetuses as young as 14 weeks gestation have the primary type of receptor that marijuana latches onto to affect the brain — a receptor that probably plays a role in normal brain development — so it makes sense that the drug could mess with the brain, according to the American College of Obstetricians and Gynecologists (ACOG).

Recently, as more states have begun to legalize recreational marijuana, researchers have published more studies about what happens when people start smoking weed while pregnant. In 2017, scientists reviewed more than 10,000 studies on cannabis and found that smoking weed during pregnancy can lead to low birth weight in the baby. However, they couldn’t isolate the effect of cannabis itself. The cause may have been the act of smoking anything at all, which increases carbon monoxide levels in the pregnant person’s blood and makes it harder for the fetus to get oxygen.

Other research suggests that frequently using weed, especially during the first and second trimesters, is associated with low birth weight — and using cannabis less than weekly may not do the same damage, according to ACOG. A new study finds that even just using marijuana during the first trimester is associated with a decrease in birth weight of a third of a pound, and if use continues into the second trimester, a decrease in head circumference of about one centimeter. The problem with low birth weight is that newborns who come out at less than 2,500g have a greater risk of some health complications such as infections and sudden infant death syndrome (SIDS).

In a 2019 study, a team of Canadian researchers found solid evidence for another negative effect of smoking weed while pregnant: preterm birth. The team looked at a registry of a large number of births in Ontario and the pregnant women’s self-reported cannabis use data. They found that 6% of non-users gave birth to preemies while 12% of users did. However, they didn’t have data on how often and in which trimester the mother’s used marijuana. To complicate matters, preterm birth findings aren’t consistent across studies, and other researchers have found that the weed either had no effect on preterm birth or that it only does when the moms-to-be used both weed and tobacco, according to ACOG.

The same Canadian team used that same Ontario birth registry to investigate whether the use of marijuana during pregnancy leads to neurodevelopmental disorders such as autism. They found that moms who used pot were about 50% more likely to give birth to an autistic child. Their analysis also showed a potential connection between marijuana use and having a baby with an intellectual disability or a learning disorder. They made these conclusions after analyzing self-reported cannabis use data and more than 500,000 live births from the start of April 2007 to the end of March 2012 in a recent study. But even though their findings are pulled from a whole lot of data, they need to be treated with caution. Although the team was able to account for socioeconomic status and some aspects of the mothers’ health, they weren’t able to include whether the moms smoked rather than ingested pot, how often they used marijuana, and when in their pregnancy they did so.

A 2022 study brings up more concerns that marijuana use during pregnancy is linked to a child’s psychopathology even a decade or more down the line. The study included more than 10,000 children in the U.S. aged 11 and 12, 391 of whom were born to a person who used marijuana before they knew they were pregnant and 208 of whom were born to someone who knowingly used weed while pregnant. The results show that marijuana use during pregnancy — particularly once a person already knows they’re pregnant — is linked to a number of issues in early adolescence, such as ADHD problems, aggressive behavior, rule-breaking behavior, social problems, obsessive-compulsive problems, and psychotic-like episodes.

There are other effects of smoking weed while pregnant that experts are even less sure about, such as whether marijuana increases the risk of stillbirth. Confounding factors such as the fact that many people who use marijuana during pregnancy also use other substances makes it difficult to tell exactly how weed affects pregnancy outcomes. But even if we don’t have the best evidence about all potential outcomes vis-à-vis weed and childbirth, we do know that the drug probably increases the risk of something harmful. With a shortage of data, it’s best to avoid it. That’s the advice of organizations such as ACOG and the American Academy of Pediatrics (AAP).

“If women are considering using cannabis during pregnancy, for example, to manage morning sickness symptoms, it will be essential to discuss this with their obstetrician or midwife,” says Daniel Corsi, Ph.D., an epidemiologist and researcher with the Canadian studies. “There are alternatives to cannabis that [are] recommended for treating morning sickness, which are safe for use in pregnancy.”

There is even less data about harms that may come from smoking weed while breastfeeding. However, there is limited evidence that THC is present in the breastmilk of people who use marijuana. Since we don’t know what we don’t know, ACOG and the AAP further advise against mixing breastfeeding and marijuana. Because when it comes to drugs and your baby, it’s better safe than sorry.